Quantitative Precipitation Forecast
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247 FOUS30 KWBC 142259 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 659 PM EDT Fri Jun 14 2024 Day 1 Valid 2242Z Sat Jun 15 2024 - 12Z Sat Jun 15 2024 ...THERE ARE MARGINAL RISK OF EXCESSIVE RAINFALL AREAS ACROSS PORTIONS OF THE EASTERN U.S. AND WESTERN PLAINS... ...2250Z Update... Given convective trends...expanded the Marginal Risk area southward and westward across portions of the northern Mid Atlantic...refer to WPC Mesoscale Precipitation Discussion 439. Similarly, the Marginal Risk area was pulled southward...refer to MPD 438. Finally...will remove the Slight risk area across southern Florida with this update given trends in radar and satellite imagery. Bann ...16Z Update... Moderate Risk inherited for the period was downgraded to a SLGT across South FL. More on the change within the "Florida" sub- heading below.... Elsewhere, the two MRGL risk areas across the Northeastern U.S and the Central Plains were generally maintained with only some minor deviations in the outlined risk areas. The setup across the Northeast U.S is unfolding currently with a cold front progressing through the eastern Great Lakes with sights further east by the second half of the forecast period. A shortwave across western PA is holding together fairly well with a core of convection propagating eastward into the northern Appalachians within the state. The forward speed of the cluster is a positive for limiting the threat for flash flooGiven radar trends and lack of ding, although some locally heavy rain within an urban setting or complex terrain could spell some localized threats in the next 2-4 hours on its progression. Guidance is keying on the shortwave entering NNJ and the NYC metro with a local enhancement of precip on its approach. There will be a general maxima within the corridor across NNJ through NYC into the Lower Hudson and western CT based on the latest probability fields, but the threat is generally maxed between 2-3", at best due to the progressive nature of the event in general. This will limit the extent of flash flooding with generally localized impacts relegated to the urban corridor. The threat over the plains remains steadfast with a quick propagation of the shortwave trough and associated convection expected later this afternoon and evening. One area to note for a general maximum is across the Caprock of eastern NM where a surface trough focus will provide an anchor for anticipated supercell development earlier this evening. This will promote slow cell motions and a higher localized threat once it develops. Otherwise, remainder of the plains maintains general continuity. ...Florida... Previous forecast was downgraded to a SLGT risk across all of South Florida with a highlighted "higher-end" SLGT located over the southeastern coastal plain in proximity to Miami up through Fort Lauderdale. Recent trends in guidance and observations favor less of a risk for widespread impacts and more isolated/scattered in terms of flash flood potential given the improvements in antecedent conditions with stream flows and urban flooding finally able to recede over the past 12+ hours. 12z soundings out of KEY and MFL indicate a dry tongue located above 600mb with much of the remnant moisture trapped beneath 700mb, maintaining PWATs within 2-2.1". Comparatively, this is a far cry from the previous days with a much greater deep moisture presence within the column allowing for a better warm cloud layer presentation and PWATs bordering on the upper 95-99th percentile climatologically. Upper pattern is in a transition phase with the deeper tropical airmass now pressing back to the west as we begin to see more influence of the surface ridge over the Atlantic flex further west and southwest over the next 24+ hours. Across the southeast Gulf, a cluster of thunderstorms will maintain intensity and begin to press southeastward as outflow driven cold pools are forecast to bring the convective cluster towards the Keys and not drive inland of the Peninsula. The main convective threat over the southern Mainland will stem from a modest LLJ located across the southern tip of the Peninsula with destabilization over land priming for convective development now through the early afternoon. Recent HREF and associated CAMs continue to signal the heaviest rain threat across the Homestead to southern Miami area with some scattered convection impacting areas north of Miami proper up through West Palm Beach. Given the environment in place, more organized, widespread heavy thunderstorms are less likely than previous periods, but the potential for locally heavy rainfall remains with probabilistic fields noting a 20-40% chance of exceeding 3" between Miami to Fort Lauderdale with the max positioned further south across the Everglades into Homestead (50-60%). Conditions will settle after 20z as the primary threat shifts north with the best chances for heavy rainfall likely around West Palm Beach up through central FL. With the general improvements in the FFG`s across South Florida, more isolated to scattered in convective coverage, and in collaboration with WFO Miami, the MDT risk was dropped to a SLGT risk with emphasis on the southeast urban corridor due to some relevant sensitivities. Kleebauer ...Previous Discussion... ...Northeast... A potent upper level shortwave will race from the Midwest to New England by early Saturday morning. The associated cold front will also move across New York State and into New England. Convection is expected to form with these forcings from Pennsylvania north and east into New England. Atmospheric moisture will be somewhat limited, and the progressive nature of the storms will limit most flooding concerns. However, given the low FFGs associated with the urban I-95 corridor and potential for cell mergers to occur as the storms move through NYC and into southern New England, the Marginal Risk area was expanded east to include almost all of southern New England except the Cape, as well as all of Long Island. Isolated instances of flash flooding are possible in the most flood-prone areas as the storms move across the area. ...Central Plains... A separate upper level shortwave will track northeastward from Arizona to the central Plains today. Lee-side cyclogenesis over eastern Colorado will tap into moisture from the LLJ with storms developing along both the warm and cold fronts across the Marginal Risk area. Despite the presence of an LLJ, moisture will be somewhat limited as northeasterly flow at 850 mb over the Gulf has largely prevented full Gulf moisture from moving into the central Plains. Thus, moisture availability will be a limiting factor for the storms. Nonetheless, there is some opportunity for raining convection in the late afternoon across New Mexico and eastern Colorado. The storms will congeal into more of an MCS overnight as the storms move a bit more quickly eastward across Nebraska and Kansas. With the ingredients for flash flooding largely separate from each other, and the area not particularly flood-sensitive, the inherited Marginal Risk remains largely unchanged with this update. Wegman Day 2 Valid 12Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... ...20Z Update... A couple changes of note were made to the previous risk areas across the Midwest/Plains, as well as Florida. Across the Midwest, the SLGT risk is well represented within the latest CAMs on both a deterministic and ensemble output, as well as a favorable probabilistic signal. Shortwave progression is fairly steady with regards to timing and magnitude with a significant convective signal focused in-of the corn belt up through the Upper Mississippi Valley. Recent ensemble trends pinpoint QPF maxima of 2-3" in multiple locations, including further south over the NE/IA border where morning convection will move overhead from the initial wave out of the plains, then spawn near the warm front and motion to the northeast later in the period. Probability fields within the general confines of the NE/IA border actually jumped significantly and became the main focus for the highest neighborhood probs for at least 3" the period (Areal coverage of 25-40%). Areas to the north where the SLGT was already in place still have reasonable outcomes with the max potential still positioned across the Upper Mississippi Valley, including the MSP metro area. Neighborhood probabilities are hovering between 10-20% for local totals exceeding 5", something that cannot be said for areas south into the corn belt. This aligns well within the previous forecast and is well representative for the SLGT continuation. Further south into the southern High Plains, the MRGL was extended to include convection initiating over the Panhandle along and just east of the dryline positioned over eastern NM. This threat is more isolated, but given the supercellular nature of the mode in question and sufficient buoyancy, a low-end flash flood risk is forecast for the late-afternoon and early-evening hours Saturday. Across Florida, the trend downward in the convective pattern will continue as the previous pattern shifts away from the Mainland with a general seabreeze and diurnally driven convective focus returning for the region. The morning may see some convection off the FL Straits and adjacent Gulf push north through the FL Keys, but the threat is not expected to produce any significant flood threat and will dissipate through the mornings hours. A MRGL risk was maintained where the seabreeze convection has the best chance for isolated flash flood concerns as it propagates inland. This is trending towards a low-end threat and the MRGL may be further adjusted, or removed pending the future model output and observed environment. Kleebauer ...Previous Discussion... ...Upper Midwest... As a strong upper level shortwave tracks northeast out of the central Plains and into the Upper Midwest, cyclogenesis occurring over the Plains will work to slow the eastward movement of the storms, while Iowa and Minnesota are on the nose of an LLJ that will be tapping increasingly deep moisture. The result will be the potential for training storms along a slow-moving warm frontal boundary over Iowa and Minnesota, with cold-frontal storms moving through overnight Saturday night having the potential for heavy rain as the nocturnal jet strengthens and PWATs associated with the LLJ increase to over 1.75 inches ahead of the cold front. Thus, there is potential for backbuilding storms as the LLJ shifts east, but Corfidi vectors will be perpendicular (westerly) to the southerly flow within the LLJ. Soils across much of Minnesota and northwestern Wisconsin are more saturated than normal for this time of year, so they will be a bit more flood prone than normal. Further, the potential for heavy rain with backbuilding convection being heightened necessitated the continuation of the inherited Slight Risk. The area was shifted a bit to the north with the latest guidance, but only nominally. This is likely to be the first of several consecutive days with threats for heavy rain for the northern Plains and upper Midwest. ...Florida... Plenty of lingering moisture and weak steering flow should allow the typical afternoon convection to develop across portions of the Florida Peninsula Saturday afternoon. Guidance is in fair agreement that the center of the Peninsula, generally north of Lake Okeechobee will be favored for the heaviest rainfall on Saturday, but it will be significantly diminished in both coverage and intensity from the rain over the Peninsula over the past few days. At this point, the heaviest rain should be sufficiently offset from the heaviest hit areas of the past few days, so flash flooding should be generally isolated. Wegman Day 3 Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AS WELL AS FOR PORTIONS OF SOUTHERN LOUISIANA... ...20Z Update... The previous SLGT risks over the Gulf Coast and Northern Plains were maintained with little change in the overall risk area(s). The best chance for flash flooding will likely occur over the Northern Plains with a focus along a stalled cold front across the NE/SD border up through southern MN. A fairly strong signal at lead for exceptional 850mb frontogenesis located over northern NE extending east-northeast into the northern Midwest. This is textbook for primed 925-850mb convergence within the confines of the front, tapping into suitable, moist airmass as PWATs settle within 1-2 deviations above normal. The signal has been most pronounced across northern NE within the Sand Hills area which generally limits widespread flash flooding concerns due to the efficiency of the soils to prevent runoff. The same cannot be said for points to the east, mainly across northeast NE into northwest IA and southwestern MN, both areas that are more prone for flash flood concerns, and will have seen a decent area of QPF in the periods prior. Any persistent convergence pattern in those areas will be subject to higher potential impact, so the SLGT risk from prior forecast was well within reason to maintain continuity. Across the Gulf Coast, signals for increasing tropical moisture advection from the Gulf have remained steady from previous forecasts with some discrepancy in the timing and location of the primary plume ejecting north around the western periphery of a broad ridge pattern to the east. In any case, the proposed advection regime would be more than conducive for heavy rain prospects, especially considering this airmass will be similar to the one that plagued FL this past week. The persistence of this pattern is less of a threat comparatively, but the chance for a slug of heavy precip is still within the ensemble QPF footprint. The SLGT risk remains given the persistence in guidance and expected pattern evolution. Kleebauer ...Previous Discussion... ...Northern Plains... The cold front following the heavy rainfall over the northern Plains on Saturday will stall out across the Slight Risk area around the SD/NE/MN/IA border region. Highly diffluent upper level low will increase lift and as the typical nocturnal jet kicks in Sunday night, convection is expected to develop over northeastern Nebraska through the evening, then track northeastward into southwestern Minnesota through the overnight. Slower moving storms and higher potential for training is expected across northeastern Nebraska, meanwhile southern Minnesota will have the benefit of a couple previous days of heavy rain over the region. The stalled out front will serve as a focus for convection, increasing the likelihood of training as the storm complex tracks northeast. With plenty of moisture available for heavy rain, a Slight Risk upgrade was introduced with this update to account for both higher potential for training over this region and favorable antecedent conditions into Minnesota. ...Southern Louisiana... A portion of the same deep tropical air mass that has haunted south Florida over the past few days will have trekked from south Florida to the central Gulf Coast by Sunday. Flow straight out of the Caribbean will increase PWATs to near 2 inches by Sunday afternoon. While convection is not expected to be too widespread, a nearly unlimited supply of moisture will allow any storms that form to be capable of very heavy rainfall rates. Both Baton Rouge and New Orleans will be threatened by storms that may move over those respective cities. Soil moisture levels are about normal for this time of year, so there has been quite a bit of recovery from the last several weeks of occasional heavy rain. Thus, this is a lower-end Slight, as more widespread convection is likely in coming days. Flood sensitive urban areas are at greatest risk of flash flooding Sunday afternoon. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt